Notes Key History Of The Atom Teacher

History of the Atom

Democritus

Proposed that all matter is composed of tiny, indivisible particles called “atomos”Today, we define an atom as the smallest part of an element that retains the chemical identity of that element

Dalton’s Atomic Theory (1807)

1) Elements are made of tiny particles called atoms.

2) All atoms of a given element are identical.

3) The atoms of a given element are different from those of any other element.

Dalton’s Atomic Theory (cont.)

4) Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms.

Dalton’s Atomic Theory (cont.)

5) Atoms are indivisible in chemical processes. That is, chemical reactions do not create or destroy atoms, they simply change the way the atoms are grouped together.

Some of Dalton’s Symbols for the Elements

Sir William Crookes (1879)

Invented the cathode ray tube and investigated electrical charges in gases.

Figure 3.7: Schematic of a cathode ray tube

John Joseph (JJ) Thomson (1903)

Discovered the negatively charged particles (0

-1e)

Proposed the “plum pudding model”

Postulated the positive particles

Ernest Rutherford (1909)

Discovered the positively charged dense central portion of the atom using his “gold foil experiment”1

1p

Figure 3.5: Rutherford’s experiment

Figure 3.6: Results of foil experiment if Plum Pudding model had been correct

Figure 3.6: Actual Results.

Figure 3.9: A nuclear atom viewed in cross section

Niels Bohr (1913)

Electrons are located in specific energy levels

Electrons move in a definite orbit around the nucleus

James Chadwick (1932)

Discovered the third subatomic particle (neutron)

Neutron has no charge and a mass of ~1 amu

10

n

Figure 3.10: Two isotopes of sodium.

Mass number (A) = protons + neutronsAtomic Number (Z) = protons

AZ X

An Ion of Sodium